the external ear, accidentally separated from the body, if accurately
replaced and fixed in position, occasionally adhere by primary union.

In the course of operations also, portions of skin, fascia, or bone, or
even a complete joint may be transplanted, and unite by primary union.

_Healing under a Scab._--When a small superficial wound is exposed to
the air, the blood and serum exuded on its surface may dry and form a
hard crust or _scab_, which serves to protect the surface from external
irritation in the same way as would a dry pad of sterilised gauze. Under
this scab the formation of granulation tissue, its transformation into
cicatricial tissue, and the growth of epithelium on the surface, go on
until in the course of time the crust separates, leaving a scar.

_Healing by Blood-clot._--In subcutaneous wounds, for example tenotomy,
in amputation wounds, and in wounds made in excising tumours or in
operating upon bones, the space left between the divided tissues becomes
filled with blood-clot, which acts as a temporary scaffolding in which
granulation tissue is built up. Capillary loops grow into the coagulum,
and migrated leucocytes from the adjacent blood vessels destroy the red
corpuscles, and are in turn disposed of by the developing fibroblasts,
which by their growth and proliferation fill up the gap with young
connective tissue. It will be evident that this process only differs
from healing by primary union in the _amount_ of blood-clot that is
present.

_Presence of a Foreign Body._--When an aseptic foreign body is present
in the tissues, _e.g._ a piece of unabsorbable chromicised catgut, the
healing process may be modified. After primary union has taken place the
scar may broaden, become raised above the surface, and assume a